Novel microwave initiated solid-state metathesis synthesis and characterization of lanthanide phosphates and vanadates, LMO4 (L = Y, La and M = V, P)

Available online at www.sciencedirect.com

Solid State Sciences 10 (2008) 1012e1019 www.elsevier.com/locate/ssscie

Novel microwave initiated solid-state metathesis synthesis and characterization of lanthanide phosphates and vanadates, LMO4 (L ¼ Y, La and M ¼ V, P) Purnendu Parhi, V. Manivannan* Department of Mechanical Engineering, Campus Delivery 1374, Colorado State University, Fort Collins, CO 80523, USA Received 18 October 2007; received in revised form 17 November 2007; accepted 25 November 2007 Available online 14 January 2008

Abstract A novel solid-state metathesis (SSM) approach represented by the reaction (LCl3 þ Na3VO4 / LVO4 þ NaCl) (L ¼ Y, La) is proposed to synthesize technologically important rare-earth phosphates and vanadates. The SSM reaction, driven in the forward direction is facilitated by the formation of high lattice energy by-product like NaCl. The structural, thermal, optical, and chemical properties of synthesized powders are determined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and diffused reflectance (DR) spectra in the UVevis range. The direct band gap of the synthesized materials was found out to be w3.5 eV for LaVO4, YVO4, YPO4 and w2.6 eV for LaPO4. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Solid-state metathesis; Lattice energy; Rare-earth phosphate; Rare-earth vanadate

1. Introduction Lanthanide metal oxides have received attention due to their potential applications in various fields such as phosphors, ion exchangers, sensors, and nonlinear optics (NLO) [1e5]. Lanthanide metal phosphates (YPO4 and LaPO4) have found applications either in the form of powders, coatings or dense sintered parts in fluorescent lamps, laminate composites, fiberematrix composites or thermal protection coatings, to name a few. Similarly, yttrium ortho-vanadate (YVO4) has been extensively used as a red phosphor with several rare-earth metal ions as dopants in cathode ray tubes (CRTs) and color televisions in powder form [6e8]. LaVO4 is well-known for catalytic as well as magnetic, luminescent and solid-state protonic conducting properties [9e11]. Synthesis of the technologically important LMO4 (L ¼ La, Y and M ¼ P, V) has been well reported in the literature. LVO4

* Corresponding author. Tel.: þ1 970 491 2207; fax: þ1 970 491 3827. E-mail address: [email protected] (V. Manivannan). 1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2007.11.038

(L ¼ La, Y) has been synthesized by high temperature solidstate reaction [12], mechanochemical [13,14], precipitation [15,16], hydrothermal [8,17], and micro-emulsion methods [17,18], etc. Similarly, LPO4 (RE ¼ La, Y) phosphates have been synthesized by solid-state reaction [19,20], mechanochemical [21,22], precipitation [23,24], hydrothermal [25,26] and micro-emulsion methods [27,28]. In general it has been observed that the chemical composition, grain size, morphology of particles as well as the associated properties of the materials are strongly influenced by the method of compound preparation. Therefore, it is important to tailor the method of synthesis for LMO4 (M ¼ V, P) compounds to have the desired optimum properties. There are several disadvantages of the above mentioned syntheses that need to be overcome. For example, synthesis of phosphovanadate materials by ball milling, where longer grinding time is applied, result in damage of the crystal particle size which causes lower luminescent efficiency. Normally, high temperature solid-state reactions produce relatively large grain size materials in addition to oxygen defect color centers. In the precipitation reaction, proper care in regulating pH has

P. Parhi, V. Manivannan / Solid State Sciences 10 (2008) 1012e1019

to be taken in order to avoid the hydroxide of the respective phase formation. Hydrothermal synthesis, in general, needs longer reaction time. Solegel and reverse micellar synthesis methods are expensive, complex and the yield is a concern. Therefore, a simplified method that can overcome the disadvantages, like long-reaction time, high temperature treatment requirement, complicated processing, use of expensive equipment, or poor yield is desired. We propose a novel solid-state metathesis reaction driven by microwave energy as a method of synthesis for these materials. Solid-state metathesis (SSM) method of synthesis is emerging as a viable alternative method of synthesis of inorganic solids and has been successfully employed in the preparation of oxides, chalcogenides, oxide superconductors, metal halides, nitrides, etc. [29e38]. We have recently shown the room temperature synthesis of Zn3(PO4)2 by solid-state metathesis method [39]. The metathesis reaction, which is a self-propagating type of reaction, where the formation of the high lattice energy by-product like NaCl drives the reaction in the forward direction, can be initiated by heating the reaction precursors by external sources of energy like using nichrome wire heating, and microwave electromagnetic waves, etc. There are several advantages in microwave heating which include the potential for energy saving, shorter processing time, improved product uniformity, yields and controlled microstructure with desired properties and formation of novel materials. Accordingly, we have chosen solid-state metathesis reactions driven by microwave energy as our synthetic approach. There are few reports of synthesis of LMO4 (L ¼ La, Y and M ¼ P, V) using microwave [40,41]. However, such reactions have either been carried out in pH adjusted solutions, ionic liquid-based solutions involving complex steps or pre-treatment like ball milling of the reactants before subjecting to microwave energy [41e47]. Microwave mediated synthesis of LaVO4 is not reported in the literature. Also, solid-state metathesis driven by microwave energy has not been reported, so far, for both LaVO4 and YVO4 compounds. The synthesis approach is characterized by high yield, less reaction time, use of less-expensive capital equipment as well as producing products with controlled morphology. In addition, to probe the driving force for the metathesis reactions we have calculated the enthalpy (DH ) and free energy changes (DG) associated with the reactions. The synthesized products have been characterized for structureeproperty relationship which included the measurement of optical band gap for these materials. 2. Experimental Na3VO4, LaCl3$7H2O, YCl3$XH2O, and Na3PO4 obtained from Alfa Aesar, USA were employed for the preparation of the title compounds. Preparation of YVO4 powder was carried out by grinding Na3VO4 and hydrated YCl3 in a molar ratio 1:1 in a pestle mortar followed by treating the reactants in a domestic microwave (2.45 GHz, operating at 1100 W power) for 10 min. The final product was obtained by washing the powder

1013

with 50 ml of deionised water and acetone and drying the product in an oven at 80  C. XRD patterns of the synthesized sample were recorded. Preparation of LaVO4 was carried out similarly by reacting hydrated LaCl3 and Na3VO4. YPO4 and LaPO4 were synthesized by the procedure mentioned above. The amorphous product obtained in both cases was subjected to further heat treatment to get the desired crystalline product. Powder X-ray diffraction (XRD) measurements were carried out with Scintag X2 diffractometer with Cu Ka radiation and Peltier detector. A scan rate of 1  /min with a step size of 0.02 was employed to obtain the XRD spectra. Search match analysis was performed using Bruker EVA software. SEM characterization was performed on the JSM-6500F, a field emission system with the In-Lens Thermal Field Emission Electron Gun (TFEG). Diffuse reflectance spectra were recorded in the wavelength range 250e2500 nm using Varian Associates Cary 500 double beam spectrophotometer equipped with Praying mantis. Employing Aluminum mesh (1.1Abs) as a rear beam attenuator compensated the reflection losses in the Praying mantis. Compressed polytetrafluoroethylene (PTFE) was used for standard calibration (100% reflectance). X-ray photoelectron spectroscopy (XPS) experiments were performed on a Physical Electronics 5800 spectrometer. This system has a monochromatic Al Ka X-ray source (ha ¼ 1486.6 eV), hemispherical analyzer, and multichannel detector. A low energy (30 eV) electron gun was used for charge neutralization on the non-conducting samples. DSC was carried out with modulated differential scanning calorimeter with liquid nitrogen accessory (TA Instruments, Model DSC 2920). The samples were hermetically sealed in Al pans and heated from room temperature to 600  C at a ramp rate of 10  C/min. 3. Results and discussion Fig. 1 shows the XRD of the as-synthesized products. XRD of the product before washing showed the presence of NaCl (marked with * in Fig. 1a). The presence of NaCl confirmed the reaction has proceeded in a metathetic pathway, as established in the literature. The metathesis reaction is represented as follows: LCl3 þ Na3 VO4 / LVO4 þ 3NaClðL ¼ La; YÞ The high lattice energy of NaCl drives the reaction in the forward direction enabling the product formation. Fig. 1b shows the XRD pattern of YVO4 after washing. Comparison of the XRD pattern with the standard JCPDS confirmed the singlephase nature of the compound. YVO4 crystallizes in the tetrag˚ , c ¼ 6.289 A ˚ , space group I41/ onal system with a ¼ 7.12 A amd. XRD of LaVO4 after washing confirms (Fig. 1c) single-phase nature of the compound in comparison with JCPDS in Ref. [48]. LaVO4 crystallizes in the monoclinic system with ˚ , b ¼ 7.286 A ˚ , c ¼ 6.725 A ˚ and b ¼ 104.85 , space a ¼ 7.047 A group P21/n. Synthesis of LPO4 (L ¼ La, Y) was performed similar to the vanadate synthesis described above using

1014

P. Parhi, V. Manivannan / Solid State Sciences 10 (2008) 1012e1019

Fig. 1. Powder X-ray diffraction pattern of sample (a) pre-washed sample, (b and c) post-washed samples showing single-phase nature for YVO4 and LaVO4, respectively.

P. Parhi, V. Manivannan / Solid State Sciences 10 (2008) 1012e1019

1015

Fig. 2. Powder X-ray diffraction pattern of sample (a) pre-washed sample, (b and c) post-washed samples showing single-phase nature for YPO4 and LaPO4, respectively.

P. Parhi, V. Manivannan / Solid State Sciences 10 (2008) 1012e1019

1016

Na3PO4 precursor. The metathesis reaction is represented as follows:

Table 1 Effect of precursors on the synthesis of LMO4 (L ¼ La, Y and M ¼ La, Y) Reactants

Microwave reaction condition (min)

Treatment

Product

LaCl3 LaCl3 LaCl3 YCl3 YCl3 YCl3 LaCl3 LaCl3 LaCl3 LaCl3

Na3VO4 NaVO3 NH4VO3 Na3VO4 NaVO3 NH4VO3 Na2HPO4 Na2HPO4 K2HPO4 Na3PO4

10 10 10 10 10 10 10 10 10 10

No

LaVO4 No reaction No reaction YVO4 No reaction No reaction LaPO4 LaPO4 LaPO4 LaPO4

YCl3 YCl3 YCl3 YCl3

NaH2PO4 Na2HPO4 K2HPO4 Na3PO4

10 10 10 10

LCl3 þ Na3 PO4 / LPO4 þ 3NaClðL ¼ La; YÞ XRD of the products is given in Fig. 2. The presence of NaCl (Fig. 2a marked with *) confirmed the solid-state reaction has proceeded in a metathetic pathway. The amorphous product obtained after washing was subjected to DSC to determine the crystallization temperature. There was no significant endothermic peak observed for the YPO4 sample. The sample heated up to 600  C showed the formation of YPO4$0.8 H2O phase and, accordingly, the sample was heated up to 800  C which showed the formation of single-phase unhydrated compound (Fig. 2b). YPO4 crystallizes in the tetragonal system ˚ , c ¼ 6.0177 A ˚ and space group I41/amd. with a ¼ 6.8817 A DSC of LaPO4 showed a sharp transition around 390  C (Fig. 3). A small quantity of the sample was heated in a furnace at this temperature. XRD analysis showed that single-phase nature of LaPO4 (Fig. 2c) in comparison to the JCPDS database [49]. LaPO4 crystallizes in the monoclinic system with ˚ , b ¼ 7.057 A ˚ , c ¼ 6.482 A ˚ and b ¼ 103.21 and a ¼ 6.8250 A space group P21/n. 3.1. Influence of precursors In order to see the effect of different precursors in LMO4 (L ¼ La, Y and M ¼ P, V) product formation, metathesis reactions were carried out by reacting metal chlorides with vanadate and phosphate precursors like NaVO3, NH4VO3, K2HPO4, NaH2PO4, and NaH2PO4. The results are remarkable in the case of LaPO4 and YPO4 where the washed products themselves showed single-phase nature. The yield of the reactions is >80% in each case. Reaction between LCl3 (L ¼ La, Y) and vanadate precursors showed unreacted precursors and there was no metathesis reaction. Details of the reaction conditions along with the products obtained are presented in Table 1. 3.2. Thermodynamic nature of reactions SSM reactions are characterized by a large enthalpy change and high adiabatic temperature. To probe the driving

No

No No No Heating at 390  C for 6 h No No No Heating at 800  C for 3 h

YPO4 YPO4 YPO4 YPO4

force for the metathesis reactions reported here, we have calculated the enthalpy (DH ) and free energy changes (DG) associated with a few of the reactions. For this purpose, we have used the thermodynamic data available in literature [50]. The final adiabatic temperature measured was 637 K for LaPO4 when Na2HPO4 precursor was used. The results showed (DG637 ¼ 79.07 kJ/mol and DH637 ¼ þ27.697 kJ/mol) that free energy favors the metathesis reaction and indeed the driving force for the metathesis reaction. SSM reactions occur so rapidly that all of the released enthalpy is essentially used to heat up the solid products, usually raising the alkali halide near or above its normal boiling point, and have been recognized as approximately adiabatic in nature. Table 2 shows the thermodynamic calculation for LaPO4. SEM image of the products obtained is shown in Fig. 4. SEM images of YVO4 and LaVO4 are shown in Fig. 4a and b, respectively. They have well-defined oval shape morphology. Both YVO4 and LaVO4 show similar morphology including the formation of nano sized particles. SEM images of the single-phase YPO4 and LaPO4 are shown in Fig. 4c and d, respectively. LaPO4 showed well-defined nanorods. The agglomeration of the particles could be due to fast nucleation and subsequent growth of the particles. XPS studies on vanadates and phosphates synthesized in this study are shown in Figs. 5 and 6, respectively. In the spectra of YVO4 (Fig. 5a) we can see peaks corresponding to V2s (630.4 eV), V2p (518.4 eV), V3p (43.2 eV), Y3s (395.2 eV), Y2p (346.66 eV), Y3d (160.0 eV), and O1s (532.0 eV). In case of LaVO4 (Fig. 5b) peaks corresponding to different Table 2 Thermodynamic data for LaPO4 synthesized by metathesis reactions 0 Composition DH298 DS0298 DG0298 DHT0 DS0T DG0T /kJ mol1 /kJ mol1 /kJ mol1 /kJ mol1 /kJ mol1 /kJ mol1

LaPO4

Fig. 3. DSC of as-prepared LaPO4 sample heated up to 600  C.

19.9

0.1357

20.53

27.697

0.1531

79.01 R

T 0 and S0T wereRcalculated using the expression, DHT0 ¼ DH298 þ 298 C0p dT T and S0T ¼ S0298 þ 298 ðC0p =TÞdT, together with the expression of C0p as a function of T. DG0T values were calculated by using the expression DG0T ¼ 0 , S0298 , C0p values were taken from Ref. [50]. DHT0  TDS0T . DH298

DHT0

P. Parhi, V. Manivannan / Solid State Sciences 10 (2008) 1012e1019

1017

Fig. 4. Scanning electron micrograph images of (a) YVO4, (b) LaVO4, (c) YPO4, and (d) LaPO4.

atoms are as follows: La3d (850.10 eV), La4d (103.7 eV), V2s (627.7 eV), V2p (515.7 eV), V3s (66.90 eV), V3p (41.3 eV), and O1s (529.3 eV). Peaks corresponding to Na1s at (1071.71) and Cl2p (194.90 eV) can also be noticed. For YPO4 (Fig. 6a) peaks corresponding to different atoms are as follows: Y3s (393.91), Y3p1/2, (313.11), Y3p3/2 (301.11), Y3d (158.7), Y4s (45.11), Y4p (25.11), P2s (190.7), P2p (133.11 eV), and O1s (531.52 eV). Na1s peaks at (1071.51) can also be found. For LaPO4 (Fig. 6b) peaks corresponding to different atoms are as follows: La3d5/2 (836.45), La4p (196.45), La4d (103.65), P2s (190.85), P2p (134.05 eV), and O1s (531.65 eV). The peaks corresponding to Na and Cl are most likely due to incomplete washing of the products. Figs. 7 and 8 show the diffuse reflectance spectra of the LVO4 and LPO4 (L ¼ La, Y) samples in the UVeviseNIR range. The diffuse reflectance data of Figs. 6a and 7a were used to calculate the absorption coefficient from the KubelkaeMunk [51,52] (KM) function defined as: FðRN Þ ¼

RN ¼

Fig. 5. XPS data of (a) YVO4 and (b) LaVO4.

a ð1  RN Þ ¼ S 2R

2

Rsample RPTFE

Here ‘‘a’’ is the absorption coefficient, and ‘‘S’’ the scattering coefficient and F(RN) is the KM function. The energy

1018

P. Parhi, V. Manivannan / Solid State Sciences 10 (2008) 1012e1019

Fig. 7. (a) Diffuse reflectance spectra of YVO4 and LaVO4 in the wavelength range 250e2500 nm. (b) Plot of F(RN) vs. E (eV) for the estimation of the optical absorption edge energy. Fig. 6. XPS data of (a) YPO4 and (b) LaPO4.

dependence of the material in the UVeviseNIR was further explored. The energy dependence of semiconductors near the absorption edge is expressed as: h  aE ¼ K E  Eg Here ‘‘E’’ is the incident photon energy (hn), ‘‘Eg’’ the optical absorption edge energy, ‘‘K’’ a constant and exponent ‘‘h’’ is dependent on the type of optical transition as a result of photon absorption [53]. The h is assigned a value of 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transition, respectively [54]. For the diffused reflectance spectra, KM function can be used instead of ‘‘a’’ for estimation of the optical absorption edge energy [53]. It was observed that a plot of F(RN) E vs. E was linear near the edge for direct allowed transition (h ¼ 1/2). The intercept of the line on the abscissa (F(RN) E ¼ 0) gave the value of optical absorption edge energy to be 3.7  0.2 eV for YVO4, 3.5  0.2 eV for LaVO4, 3.5  0.2 eV for YPO4 and 2.6  0.2 eV for LaPO4. Figs. 6b and 7b show the plot of the

same. The diffuse reflectance spectra for direct band gap orthorhombic (b) [55] Ta2O5 prepared by heating Ta metal in air are also recorded for comparison. The value of optical absorption edge energy for the indirect allowed transition for Ta2O5 was found to be 4.0  0.2 eV, which is consistent with those seen for the b-Ta2O5 reported [56]. 4. Conclusions LMO4 (L ¼ La, Y and M ¼ P, V) has been successfully synthesized by a solid-state metathesis approach driven by microwave energy. The influence of various phosphate and vanadate precursors on the formation of final products has been investigated. Highly crystalline single-phase products with controlled morphology have been obtained which were characterized for structural properties including optical band gap of the material. The proposed synthesis approach is simple, cost-effective and has the potential for easy scale-up. Thermodynamic calculations show that free energy favors the metathesis reaction and indeed the driving force for the metathesis involving the formation of NaCl.

P. Parhi, V. Manivannan / Solid State Sciences 10 (2008) 1012e1019

Fig. 8. (a) Diffuse reflectance spectra of YPO4 and LaPO4 in the wavelength range 250e2500 nm. (b) Plot of F(RN) vs. E (eV) for the estimation of the optical absorption edge energy.

Acknowledgments The authors would like to acknowledge Prof. Allan Kirkpatrick, Head of the Department, Mechanical Engineering, Colorado State University, for his continued help, encouragement and support. References [1] H.Y. Kang, L.M. Meyer, R.C. Haushalter, A.L. Schweitzer, L. Zubieta, J.L. Dye, Chem. Mater. 8 (1996) 43. [2] M.S. Whittingham, Mater. Res. Bull. 13 (1978) 959. [3] R.E. Sykora, K.M. Ok, P.S. Halasyamani, D.M. Wells, T.E. AlbrechtSchmitt, Chem. Mater. 14 (2002) 2741. [4] D. Papoutsakis, J.E. Jackson, D.G. Nocera, Inorg. Chem. 35 (1996) 800. [5] J.L. Bideau, D. Papoutsakis, J.E. Jackson, D.G. Nocera, J. Am. Chem. Soc. 119 (1997) 1313. [6] C.-H. Huang, J.-C. Chen, C. Hu, J. Cryst. Growth 211 (2000) 237. [7] R.A. Fields, M. Birnbaum, C.L. Fincher, Appl. Phys. Lett. 51 (1987) 1885. [8] K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) 10129.

1019

[9] Z.M. Fang, Q. Hong, Z.H. Zhou, S.J. Dai, W.Z. Weng, H.L. Wan, Catal. Lett. 61 (1999) 39. [10] B.C. Chakoumakos, M.M. Abraham, L.A. Boatner, J. Solid State Chem. 109 (1994) 197. [11] Y. Oka, T. Yao, N. Yamamoto, J. Solid State Chem. 152 (2000) 486. [12] T. Sivakumar, J. Gopalakrishnan, Chem. Mater. 14 (2002) 3984. [13] T. Tojo, Q. Zhang, F. Saito, J. Alloys Compd. 427 (2007) 219. [14] S. Erdei, J. Mater. Sci. 30 (1995) 4950. [15] E.A. Arbit, V.V. Serebrennikov, Russ. J. Inorg. Chem. 10 (1965) 220. [16] E.J. Baran, I.L. Botto, J. Inorg. Nucl. Chem. 40 (1978) 1603. [17] W. Fan, X. Song, S. Sun, X. Zhao, J. Solid State Chem. 180 (2007) 284. [18] L.D. Sun, Y.X. Zhang, J. Zhang, C.H. Yan, C.S. Liao, Y.Q. Lu, Solid State Commun. 124 (2002) 35. [19] M.-Z. Su, J. Zhou, K.-S. Shao, J. Alloys Compd. 207 (1994) 406. [20] J.M. Nedelec, D. Avignant, R. Mahiou, Chem. Mater. 14 (2002) 651. [21] J.A. Diaz-Guillen, A.F. Fuentes, S. Gallini, M.T. Colomer, J. Alloys Compd. 427 (2007) 87. [22] Y. Hikichi, K. Yogi, T. Ota, J. Alloys Compd. 224 (1995) L1. [23] X. Wang, M. Gao, J. Mater. Chem. 16 (2006) 1360. [24] S. Lucas, E. Champion, D. Bregiroux, D. Bernache-Assollant, F. Audubert, J. Solid State Chem. 177 (2004) 1302. [25] Y. Zhang, H. Guan, Mater. Res. Bull. 40 (2005) 1536. [26] K. Ioku, T. Okada, E. Okano, K. Yanagisawa, N. Yamasaki, Phos. Res. Bull. 5 (1995) 71. [27] M. Cao, C. Hu, Q. Wu, C. Guo, Y. Qi, E. Wang, Nanotechnology 16 (2005) 282. [28] J.M. Nedelec, C. Mansuy, R. Mahiou, J. Mol. Struct. 651 (2003) 165. [29] P.R. Bonneau, R.F. Jarvis, R.B. Kaner, Nature 349 (1991) 510. [30] P. Parhi, A. Ramanan, A.R. Ray, Mater. Lett. 58 (2004) 3610. [31] P. Parhi, A. Ramanan, A.R. Ray, Mater. Lett. 60 (2006) 218. [32] T.K. Mandal, J. Gopalakrishnan, Chem. Mater. 17 (2005) 2310. [33] J. Gopalakrishnan, T. Sivakumar, K. Ramesha, V. Thangadurai, G.N. Subbanna, J. Am. Chem. Soc. 122 (2000) 6237. [34] A.M. Nartowski, I.P. Parkin, M. MacKenzie, A.J. Craven, I. Macleod, J. Mater. Chem. 9 (1999) 1275. [35] L. Rao, E.G. Gillan, R.B. Kaner, J. Mater. Res. 10 (1995) 353. [36] R.E. Treece, J.A. Conklin, R.B. Kaner, Inorg. Chem. 33 (1994) 5701. [37] D. Mingos, P. Michael, Adv. Mater. 5 (1993) 857. [38] B. Vaidhyanathan, M. Ganguli, K.J. Rao, Mater. Res. Bull. 30 (1995) 1173. [39] P. Parhi, V. Manivannan, Mater. Res. Bull., in press. [40] K. Uematsu, K. Toda, M. Sato, J. Alloys Compd. 389 (2005) 209. [41] H.Y. Xu, H. Wang, Y.Q. Meng, H. Yan, Solid State Commun. 130 (2004) 465. [42] H. Xu, H. Wang, H. Yan, J. Hazard. Mater. 144 (2007) 82. [43] K. Uematsu, A. Ochiai, K. Toda, M. Sato, J. Alloys Compd. 408e412 (2006) 860. [44] K. Uematsu, K. Toda, M. Sato, Chem. Lett. 8 (2004) 990. [45] G. Buhler, C. Feldmann, Angew. Chem. Int. Ed. (2006) 4864. [46] G. Buhler, M. Stay, C. Feldmann, Green Chem. 9 (2007) 924. [47] G. Buhler, C. Feldmann, Appl. Phys. A 87 (2007) 631. [48] JCPDS Card No. 72-274, 70-2392, International Centre for Diffraction Data, Newton Square, PA, (2000). [49] JCPDS Card No. 84-335, 84-600, International Centre for Diffraction Data, Newton Square, PA, (2000). [50] M. Binnewies, E. Milke, Thermochemical Data of Elements and Compounds, second ed. Wiley-VCH, Weinheim, 2002. [51] P. Kubelka, F. Munk, Z. Tech. Phys. 12 (1931) 593. [52] G. Kortum, Reflectance Spectroscopy Principles, Methods, Applications, Spinger-Verlag, New York, 1969. [53] D.G. Barton, M. Shtein, R.D. Wilson, S.L. Soled, E. Iglesia, J. Phys. Chem. B 103 (1999) 630. [54] J. Tauc, R. Grigorov, A. Vancu, Phys. Status Solidi 15 (1966) 627. [55] B.R. Sahu, L. Kleinman, Phys. Rev. B 69 (2004) 165202. [56] W.H. Knausenberger, R.N. Tauber, J. Electrochem. Soc. 129 (1973) 927.